Reposted from the DWAITAS forum as it's probably more relevant for Cthulhu Britannica and other Pulp games.

"Will Doctor Gandalf Grey, believed to be travelling near Inverness, please contact Addenbrooke’s Hospital in Cambridge where his uncle is dangerously ill".

Sitting at a pub table musing about lunch options Gandalf was startled by the message. Not just because he didn’t have an uncle, and wouldn’t for several decades, but for the sheer unexpectedness of the message, interrupting as it did a pleasant and quiet day.
It was one of the prearranged codes he’s set up with members of his circle of acquaintances, making use of the willingness of the BBC to transmit ‘SOS’ messages over the air and their lack of verification of them.

The use of ‘uncle’ meant the matter was urgent but that wasn’t what worried him; the mention of Addenbrookes, the hospital founded by a necromancer, meant the matter, whatever it was, had the potential to cause extensive death.

He’d need to phone the Hourglass Club immediately, collect the message waiting there for him, and find out what was up.

Lunch would have to wait while he arranged a trunk call.

In the decades before mobile phones the BBC would assist people by reading out ‘SOS’ messages after news broadcasts and before other programmes.
It all started in March of 1923 in Birmingham, when the Corporation was still merely the British Broadcasting Company; the local station sent out an appeal for help in finding a missing boy. Inspired by the success (the six-year old was found and returned safely to his family) the first Managing Director of the BBC, the famous Lord Reith, created an arrangement between the company and government and emergency services. For more than seventy years the SOS message service, later limited to Radio 4 after it replaced the Home Service, alerted the public to ill relatives, missing people (and once a missing pelican), unexploded bombs, infected animals, lost poisons and other matter. Even in one case and appeal for a wet nurse (Norfolk in 1937).

They were very short, and clinical. "Will Mr and Mrs Little, last heard of eight months ago in the Birmingham area, head to Leeds General Infirmary where Mrs Little's mother is dangerously ill", for example. Less than thirty seconds and composed using a standard formula. Alas for the curious the details, like what happened afterwards, weren't revealed.

I was introduced to the service in fiction years before before I actually heard such a message; by the time the service petered out in the mid-1990s (blame cellular telephony) there were about 25 such messages each year. In Biggles Hits the Trail [set around 1935] the adventure opens with the following;

A voice, faint at first, but rapidly increasing in volume was speaking ‘...few minutes late. Now before we begin here is an S.O.S.’

‘Oh, confound these S.O.S’.s,’ grumbled Algy. ‘I’ve never heard of anyone answering....’
The words died away on his lips as the voice of the announcer continued.

‘Will Major James Bigglesworth – B-I-double G-L-E-SW-O-R-T-H, Major James Bigglesworth, last heard of at Brooklands Aerodrome, go at once to Brendenhall Manor, Buckinghamshire, where his uncle Professor Richard Bigglesworth, is dangerously ill.’

Algy stared at the instrument. ‘Well, I’m —’

‘Shut up – he hasn’t finished,’ snapped Biggles.

‘I am requested to add,’ continued the voice, ‘that if Major Bigglesworth receives this message, and goes to Brendenhall, he is advised to exercise the same caution as on the occasion of his last visit – whatever that may mean.... And now we are going over to the Albert Hall for —’

What follows is a classic Pulp adventure complete with a Sinister Society plotting World Domination using a Death Ray.

A fascinating possibility for entangling players in an adventure in the period from 1923-93, whether Doctor Who, Call of Cthulhu or other Pulp type.

Rules for SOS messages [from the 1967 edition 'BBC Year Book']
SOS and police messages are in certain circumstances included in BBC broadcasts.For Relatives of Sick Persons. Such SOS messages are broadcast only when the hospital or doctor certifies that the patient is dangerously ill and when all other means of communications have failed. Normally the full name of the person sought, and the relationship, must be given. The message is broadcast only if the patient is asking to see a relative or the doctor considers that this would be beneficial.For Missing Persons and For Witnesses of Accidents. Only official requests originated by the police are considered.Appeals for Special Apparatus, Foods, or Drugs for treatment of rare diseases will be broadcast only at the request of major hospitals and after all other means of obtaining them have failed.
Requests may be made by personal call, by letter, or by telephone.For Travellers Abroad. It is also possible in circumstances of real urgency for SOS messages to be broadcast in countries abroad by radio organizations which are members of the European Broadcasting Union. These
messages would be broadcast in an attempt to reach people travelling abroad who are urgently wanted at home. The rules, in principle, are exactly the same as those which apply to SOS messages broadcast in the United Kingdom. Requests of this kind, which must come from doctors or hospitals, cannot be considered unless all other means of contacting the person who is wanted have been tried and have failed.
Messages are broadcast once only and cannot be repeated.
There is no charge for broadcasting SOS messages.

Game use.
1. The party hear an SOS that mentions the name (or alias) of an antagonist, or someone they're looking for. Finally they have a idea where s/he is, or is likely to be. Do they hare off in search of the person? Is it a trap? And if so, is it set for them or the person they;re seeking?

2. The party hears a SOS that's directed at them. Who sent it? Is it genuine? Or a trap? Even if they miss the message itself someone else could heard it and pass it along (as happened with the real service).

3. The service could also be used, carefully and infrequently, as a system for notifying someone (like Gandalf) that Something Is Up. Of course the message is public and broadcast across the BBC network so others could easily hear it too.

4. The BBC are concerned by certain SOS messages that they have, in good faith, broadcast recently but which were hoaxes. Then someone notices that there seems to be a link to strange events in the locations mentioned in the messages. The PCs investigate.